The structure of the avian lung is very different from that of an alveolar lung and its function as an O2 and CO2 exchanger has been described by a cross-current model. Theoretically, cross-current lungs are more efficient at oxygenating blood and eliminating CO2 than alveolar lungs, but experiments have indicated that bird lungs do not usually operate at such optimal levels of efficiency. First, I propose to test the cross-current model for inert gases in birds because this gives more information about the lung than just O2 and CO2. Once the appropriate model for avian gas exchange is described, I will quantify and define factors important in pulmonary gas exchange in birds. Distrubutions of ventilation and perfusion will be determined with the Multiple Inert Gas Elimination Technique (MIGET). The morphological basis of such imhomogeneities will be investigated with electron microscopy of lungs rapidly frozen under various physiologic conditions. The size of the functional unit of gas exchange in birds will be described as the size of the pulmonary vessels obstructed by beads just large enough to produce changes in V/Q distributions determined by the MIGET. Factors affecting the distribution of ventilation will be examined with a helium wash-through on unidirectionally ventilated birds. Airway diffusion limitations will be evaluated by comparing washouts of pairs of inert gases with similar solubilities, but different molecular weights infused in birds. Membrane diffusion limitations will be quantified at various levels of inspired O2 tension as the difference between arterial PO2 predicted by a model of avian O2 and CO2 exchange, considering the degree of V/Q inequality measured by the MIGET, and the arterial PO2 actually measured. Assumptions about distributions of diffusing capacity and blood flow required in the model will be investigated in rapidly frozen lungs with electron microscopy. Studying the physiology and morphology of factors determining the efficiency of pulmonary gas exchange in birds will broaden our basic knowledge of these phenomena which are also the primary determinants of gas exchange in man during health and disease.